In recent years, as a scaling process for images, a “nearest neighbor method” for selecting a pixel of original image data which is nearest to the pixel of interest after scaling is known.
Also, a process such as a “bi-cubic method” or the like is well known.
However, since the “nearest neighbor method” uses only the nearest pixel data, deterioration of an image is considerable, e.g., moiré is generated, and pixels look coarse upon up-scaling.
On the other hand, the “bi-cubic method” can assure higher image quality than the “nearest neighbor method”. However, since it maintains high resolution, when a document with periodicity, e.g., a printed document, is scanned and undergoes an image process upon down-scaling, moiré fringes are generated and the image quality deteriorates.
Especially, when a binary image undergoes scaling, deterioration of an image is considerable. When a binary image is scaled at a scale near 100%, even an image of a document having no periodicity suffers periodic density nonuniformity, resulting in poor appearance.
In another method, a binary image may be temporarily converted into a multi-valued image, which may then undergo scaling. However, with such method, the storage size of an image storage device and the image transfer size increase.